wifi: rtw89: mcc: refine filling function of start TSF

Since tob (time offset behind) could be negative, change the type of tob in
microsecond to s32. And, refine accumulation with while-loop by calculation
with roundup_u64(). Besides, as long as one of the MCC roles is GO, use the
short MCC start time.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250422014620.18421-12-pkshih@realtek.com